1. Field of the Invention
The present invention relates to an ink jet printer, and, more particularly, to a print media feed system for an ink jet printer.
2. Description of the Related Art
An ink jet printer includes a paper or print media feed system which transports the print media, one sheet at a time, from a supply source such as a paper tray to a print zone where ink is jetted onto the print medium using an ink jetting printhead. The ink jetting printhead includes a plurality of ink jetting nozzles from which a selected color ink is jetted onto the print medium. Print quality can be improved if the print medium is allowed to be as close as possible to the print nozzles of the printhead. The clearance distance between the front side of the print medium and the print nozzles of the printhead is important because all ink jetting printheads exhibit a certain amount of satellites (smaller stray drops of ink) that are ejected slightly off trajectory from the main drop. If the clearance distance between the front side of the print medium and the ink jetting nozzles is small, the stray drops will be minimized.
However, the clearance distance between the ink jetting nozzles and the front side of the print medium can only be minimized to a certain extent. If the clearance distance is too small, cockle (waviness) in the print medium caused by wet ink may cause the front side of the print medium to touch the printhead in the area of the ink jetting nozzles. When the print medium is touching the printhead, the ink drops of course cannot be properly jetted onto the front side thereof for formation of a print image.
Conventional systems which control the clearance distance between the print medium and the printhead are of two basic types. The first type uses a back side control surface which is disposed at a predetermined distance away from the printhead. The print media is forced against the back side control surface. The distance between the front side of the print medium and the printhead thus varies dependent upon the thickness of the print medium. Since the back side control surface is fixed, the clearance distance between the front side of the print medium and the printhead must be sufficiently large such that the print medium will not contact the printhead in the event that cockling of the print medium occurs. Decreasing the clearance distance to improve print quality may result in the print medium contacting the printhead, which is not desirable as described above.
The second general type of system used to control the clearance distance between the print media and the printhead biases the print media against a front side control surface, as opposed to a back side control surface. The front side control surface may either be movable in transverse directions along with the printhead, or fixed and extend across the width of the print media. In either event, the clearance distance between the edge of the front side control surface and the print media is fixed and does not change, regardless of the type of print media used during printing.
For example, U.S. Pat. No. 5,648,807 (Saito, et al.) disclose an ink jet printer (FIG. 3A) having a paper feed roller 330 which is engaged by a pinch roller 350. Pinch roller 350 is rotatably attached to the distal end of a paper guide 53 which is suspended from a rear, fixed frame 130 using a spring 52 so that paper guide 53 rotates about a fulcrum point 51. Frame 130 not only interconnects with paper guide 53 as shown in FIG. 3A, but also substantially forms an enclosure which carries the plurality of gears, rollers, etc. (FIG. 18). As shown in FIG. 27, a lower end of rear frame 130 is attached to and carries a pressing member 140 which is disposed above feed roller 330. Because of the fixed nature of frame 130, pressing member 140 is always "located at a slightly lower position from a tangent T to both feed roller 330 and transport roller 381, and is arranged to press paper P downward." Because of the fixed and immovable nature of frame 130 and pressing member 140, pressing member 140 does not move with or relative to pinch roller 350 carried by paper guide 53, but rather is fixed in a stationary position.
What is needed in the art is a print media feed system which overcomes the problems associated with a fixed front side media control surface and a fixed back side media control surface.